Include a CRC32-IEEE checksum in log lines (#4478)

Adds a CRC32-IEEE checksum to our log lines. At most this adds 8 bytes per line, and at least adds 2 bytes. Given this a relatively minor change I haven't bothered flagging it, although if we have anything in place that assumes the current structure of log lines we may want to add a flag in order to prevent immediate breakage before things can be altered.

Fixes #4474.

log/log.go

@@ -1,9 +1,12 @@
 package log
 
 import (
+	"encoding/base64"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"hash/crc32"
 	"log/syslog"
 	"os"
 	"path"
@@ -117,6 +120,15 @@ type bothWriter struct {
 	clk         clock.Clock
 }
 
+func LogLineChecksum(line string) string {
+	crc := crc32.ChecksumIEEE([]byte(line))
+	// Using the hash.Hash32 doesn't make this any easier
+	// as it also returns a uint32 rather than []byte
+	buf := make([]byte, binary.MaxVarintLen32)
+	binary.PutUvarint(buf, uint64(crc))
+	return base64.RawURLEncoding.EncodeToString(buf)
+}
+
 // Log the provided message at the appropriate level, writing to
 // both stdout and the Logger
 func (w *bothWriter) logAtLevel(level syslog.Priority, msg string) {
@@ -126,6 +138,8 @@ func (w *bothWriter) logAtLevel(level syslog.Priority, msg string) {
 	const red = "\033[31m\033[1m"
 	const yellow = "\033[33m"
 
+	msg = fmt.Sprintf("%s %s", LogLineChecksum(msg), msg)
+
 	switch syslogAllowed := int(level) <= w.syslogLevel; level {
 	case syslog.LOG_ERR:
 		if syslogAllowed {

log/log_test.go

@@ -106,8 +106,8 @@ func ExampleLogger() {
 	impl.AuditErr("Error Audit")
 	impl.Warning("Warning Audit")
 	// Output:
-	// E000000 log.test [AUDIT] Error Audit
-	// W000000 log.test Warning Audit
+	// E000000 log.test 46_ghQg [AUDIT] Error Audit
+	// W000000 log.test 9rr1xwQ Warning Audit
 }
 
 func TestSyslogMethods(t *testing.T) {